The invention relates to a transponder for the contactless inductive communication with a base station, comprising an antenna resonant circuit adapted to receive a modulated HF signal supplied by the base station and to supply said signal to signal processing means, and signal processing means for processing a received modulated HF signal and for supplying a data signal and a clock signal, the data signal comprising data contained in the modulated HF signal and the frequency of the clock signal being derived from the frequency of the HF signal, and data processing means arranged to receive the data signal and the clock signal and adapted to process the data signal, the processing speed of the data processing means being dependent upon the clock signal and the data processing means being arranged to receive reset information for terminating the processing of the data signal.
The invention further relates to an integrated circuit for realizing a transponder constructed to provide contactless inductive communication with a base station, which transponder comprises an antenna resonant circuit adapted to receive a modulated HF signal supplied by the base station and to supply said signal to signal processing means, comprising signal processing means for processing a received modulated HF signal and for supplying a data signal and a clock signal, the data signal comprising data contained in the modulated HF signal and the frequency of the clock signal being derived from the frequency of the HF signal, and data processing means arranged to receive the data signal and the clock signal and adapted to process the data signal, the processing speed of the data processing means being dependent upon the clock signal and the data processing means being arranged to receive reset information for terminating the processing of the data signal.
Such a transponder of the type defined in the first paragraph and an integrated circuit of the type defined in the second paragraph are known, for example from the document U.S. Pat. No. 5,345,231 A. The known transponder comprises an antenna resonant circuit having an operational link with an antenna resonant circuit of a base station for the contactless inductive communication via a modulated HF signal. The modulated HF signal is generated by means of the base station and has an operating frequency.
The received modulated HF signal occurring in the antenna resonant circuit of the transponder during operation of the base station can be applied to a power supply stage, which derives a direct voltage for the power supply of the stages present in the transponder from the received HF signal. The power supply stage further includes a reset stage which supplies reset information to the stages in the transponder when said voltage drops below a minimum voltage which guarantees a reliable operation of the transponder, in order to terminate the operation of the transponder and thus the processing of a data signal.
In a reception mode of the transponder a received modulated HF signal occurs in the antenna resonant circuit of the transponder, which signal carries, in modulated form, digital data to be transmitted from the base station to the transponder. The modulated HF signal can be applied to signal processing means adapted to generate a clock signal having the frequency of the HF signal. The clock signal can be applied to data processing means formed by a microcomputer and defines the system clock and hence the processing speed in the data processing means, so that the processing speed of the data processing means depends on the clock signal.
Furthermore, the signal processing means are adapted to demodulate the received modulated HF signal and to supply a data signal to the data processing means. In a data processing mode the data processing means can process the digital data present in the data signal, after which the processed digital data can be stored in a memory, which can also result in data already stored in a memory being changed.
In a transmission mode of the transponder the digital data processed by the data processing means can be supplied as a data signal to the signal processing means. The signal processing means produce a load modulation of a non-modulated HF signal via the antenna resonant circuit of the transponder and an antenna resonant circuit of a base station, thereby enabling the digital data processed in the transponder to be transmitted inductively to the base station in a contactless manner.
The data which can be transmitted from the base station to the transponder and from the transponder to the base station and which can be stored in the memory of the transponder are nearly always security-related data, which represent for example amounts of money and which should be changed only by an authorized person. In the base station such security-related data are encoded by means of a digital key and after the transmission to the known transponder in the data processing means of this transponder they are decoded in the data processing means of this transponder by means of a digital key stored in a memory of the transponder, so that a high data security is achieved.
Owing to the recent development of new measurement technologies it has been found that by means of an intricate though possible measurement process applied to or in the transponder, in which an HF signal whose frequency is low in comparison with the operating frequency is transmitted to the transponder, as a result of which the data processing means have a comparatively low processing speed, security-related data which are available in non-encoded form in the transponder and which are transmitted between the data processing means and the memory of the transponder via an electrically conductive connection during this processing can be detected under certain circumstances with the aid of said electrically conductive connection. Such a measurement process can also be carried out without permission by persons who are not authorized to detect such security-related data. As a result of this recent development a desired high data security for security-related data is no longer attainable with a sufficient degree of reliability by means of the known transponder for contactless communication, which is an undesirable effect.